Research paperMicrostructural and mechanical properties of poly(sialate-siloxo) networks obtained using metakaolins from kaolin and halloysite as aluminosilicate sources: A comparative study
Introduction
The most important minerals contained in the kaolin group are kaolinite and halloysite. Both minerals are dioctahedral 1:1 layer silicates and these minerals are denoted hydrated aluminosilicate materials. Halloysite is a natural nanoscroll material which exists in two forms namely the hydrated and dehydrated forms with the chemical formulas Al2Si2O5(OH)4.2H2O and Al2Si2O5(OH)4, respectively (Nicolini et al., 2009). This implies that halloysite has a similar composition like kaolinite except that it contains additional water molecules between their layers. This justifies the fact that the basal distance of the hydrated form is around 10 Å whereas the one of dehydrated form is around 7 Å. The dehydrated form of halloysite also called metahalloysite is identical to the one of kaolinite. This means that the identification of halloysite 7 Å is difficult (Brindley, 1980). The shape of the kaolinite particles is hexagonal plate whereas halloysite displays tubular, spherical and cone-shaped morphologies (Kitagawa, 1976). According to Senoussi et al. (2016), the tubular form is predominant in the structure of halloysite motivating to use it as an absorbent. According to Sudo (1953), halloysite possesses fine particles size (between 0.05 and 0.50 μm) and Hong et al. (2009) reported that kaolinite has a coarse-grained ranging from 1 to 2 μm in its structure. Nelson and Hendricks (1943) and Dyal and Hendricks (1950) reported that the value of the specific surface area measured by nitrogen and ethane adsorption is in the range 7–30 m2/g for kaolinite and about 45 m2/g for halloysite indicating that halloysite has a higher specific surface area compared to the one of kaolinite. Halloysite has been already used by several authors either as an adjuvant and an additive to alter the properties of Portland cement pastes and concretes. For example, Farzadnia et al. (2013) used it as an adjuvant to investigate the effect of halloysite on the properties of cement mortars. In this work, 1, 2 and 3% of halloysite was used to substitute the Portland cement. They reported that the compressive strength and gas permeability of the final product containing 2 and 3% of halloysite ameliorated the properties of mortar from Portland cement up to 56 and 24%, respectively. Whereas Zapała-Sławeta (2017) used metakaolin from halloysite as an additive, i.e., 0, 5, 10, 15 and 20% to substitute Portland cement CEM I 42.5R in order to study the effect of metakaolin from halloysite on the alkali-aggregate reaction in concrete. They demonstrated that it is possible to mitigate alkali-silica reaction and lower expansion of mortar bars with reactive aggregate to a safe level of not more than 0.1% at 14 days by adding 20% of metakaolin from halloysite to Portland cement. The literature indicates that geopolymer cements were mostly produced using fly ash, slag, metakaolin from kaolin, volcanic ash and so on. In our point of view, the aluminosilicate named halloysite could be used for producing geopolymer cements with higher mechanical properties. For example, Zhang et al. (2012) used this material as a secondary mineral to study its influence in kaolin on the properties of geopolymer cements. They concluded that halloysite present in kaolin improved the reactivity of the metakaolin and therefore have a positive effect on the properties of metakaolin-based geopolymer materials. Up to now, some researchers such as Zivica et al. (2014) used halloysite as a raw material for synthesizing geopolymer cements. In this work, the author's calcined halloysite at 650 °C for 4 h. They reported that the compressive strength of the hardening geopolymer cement paste cured at room temperature for 24 h and compacted in the fresh state with a uniaxial pressure of 300 MPa was 76.2 MPa whereas the one without compacted was 0.03 MPa. Kaze et al. (2018) also used metakaolin from halloysite for producing geopolymer cements. These authors used sodium waterglass containing sodium hydroxide with molar concentrations 8, 10, 12 and 14 M. The volume ratio sodium hydroxide/sodium silicate was kept constant at equal to 1/1. They reported that the maximum compressive strength is around 27.5 MPa. We assume that the properties of halloysite mentioned above worth particular attention especially for its use in the synthesis of geopolymer cements. Several researchers attempted to compare the properties of geopolymer cement using metakaolin and other aluminosilicate sources. For examples, Robayo-Salazar et al. (2016) describe the effect of metakaolin on the microstructure and compressive strength of geopolymeric systems based on natural volcanic pozzolans. Douiri et al. (2017) compared the structural and dielectric of geopolymers prepared with metakaolin and Tunisian natural clay. Sudagar et al. (2018) investigated the influence of cork waste residue on metakaolin-zeolite-based geopolymers. Belmokhtar et al. (2018) investigated the comparison between the structural and textural properties of geopolymer cement from industrial sludge and commercial kaolin. Bhardwaj and Kumar (2019) attempted to compare the properties of geopolymer and alkali-activated slag concrete comprising waste foundry sand. Marinkovi et al. (2017) determine the appropriate functional unit for concrete mixes with possible different performances and compare these impacts for different concretes. Despite these works, no research has been done on comparing the properties of geopolymer cement using metakaolins from kaolin and halloysite. Rice husk is one of the most widely available agricultural wastes in many rice-producing countries around the world. Burning of rice husk in ambient atmosphere leaves a residue, called rice husk ash (RHA). The ash content of around 85–97 wt% of amorphous silica (Rozainee et al., 2008). RHA is highly porous and lightweight, with a very high external surface area. The presence of a high amount of silica makes it a valuable material for use in industrial application. This low-value silica-rich waste was used by several researchers (Tchakouté et al., 2016a, Tchakouté et al., 2016b) for preparation of sodium waterglass.
The aim of this work is to compare the microstructural and mechanical properties of poly(sialate-siloxo) networks obtained using metakaolins from kaolin and halloysite as aluminosilicate sources. The chemical reagents used are a commercial sodium waterglass and a sodium waterglass from rice husk ash. The comparative study of poly(sialate-siloxo) networks using metakaolins from kaolinite and halloysite was done by the determination of their compressive strengths and apparent densities. The fragments of samples obtained after compressive strengths measurement were used for scanning electron microscopy (SEM) observations. The powders were used for identifying the crystalline phases and functional groups in the obtained geopolymer cements using XRD diffractometry and IR spectroscopy, respectively.
Section snippets
Materials
The kaolin and halloysite clay minerals used in this work were collected from Mayouom and Balengou, respectively, in the Western region of Cameroon. Metakaolin denoted MK-BALCO from standard white kaolin provides by BAL-CO, Modena (Italy) was also used. This company obtained metakaolin by calcination of white kaolin at 700 °C for 4 h and commonly used for producing glazed ceramic tile. The chemical formula of the obtained metakaolin was 5.4SiO2.4Al2O3 (Kamseu et al., 2014). The kaolin and
Chemical compositions of kaolin and halloysite
The chemical compositions of halloysite (HA) and kaolin (MY33) reported by Njopwouo et al. (1987) and Njoya et al. (2006), respectively are given in Table 1. These materials are mainly constituted of silicon dioxide (56 wt% for HA and 46.59 wt% for MY33) and alumina (29.30 wt% for HA and 34.46 wt% for MY33). Halloysite and kaolin have 2.60 and 1.05 wt% of Fe2O3 content, respectively. The absence of the K2O in the structure of halloysite could be associated with the absence of micaceous or
Conclusion
Metakaolins from halloysite and kaolin were used as aluminosilicate sources in this work to compare the mechanical and microstructural properties of poly(sialate-siloxo) networks. The XRD pattern of metakaolin from halloysite indicates the lower broad hump structure compared to those of metakaolins from kaolin. The infrared spectrum and XRD pattern of metakaolin from halloysite show the residual peaks of halloysite in its structure. The micrograph images of metakaolin from halloysite present
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Dr. Tchakouté Kouamo Hervé gratefully acknowledges the Alexander von Humboldt Foundation for its financial support this work under grant N° KAM/1155741 GFHERMES-P. The authors would like to thank Dr. Valerie Petrov for SEM observations.
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